During a call within a mobile communication system, a communication unit (CU) maintains a channel with a system antenna, or "system node", which provides communication services to the area in which the CU is located. A system node can be, for example, a ground-based cellular base station or a satellite. When the CU transitions into an area covered by a different system node, the CU must be handed off to the different system node or the call will be dropped.
As used herein, the system node whose coverage area the CU is leaving (i.e., the node from which the CU is being handed off) is referred to as the "losing node". The system node whose coverage area the CU is entering (i.e., the node to which the CU is being handed off) is referred to as the "gaining node". Handoff between a losing node and a gaining node is referred to herein as an "inter-node handoff".
A CU transitions into the coverage area of a gaining node due to the relative motion of the system nodes to the CU. For example, where system nodes are stationary (e.g., ground-based system nodes or satellite system nodes with earth-fixed coverage areas), the CU would have physically to relocate to enter the coverage area of the gaining node. Where the coverage areas of system nodes move relative to the surface of the earth (e.g., non-geosynchronous satellite system nodes with satellite-fixed coverage areas), the CU does not need to move to enter the coverage area of the gaining node. In such a case, the coverage area moves over the CU.
Prior art methods of handing off a CU from a losing node to a gaining node use either "make-before-break" or "break-before-make" handoff sequences. A "make-before-break" handoff sequence requires a CU to be capable of sustaining two simultaneous traffic channels during handoff: one with the losing node and one with the gaining node. When a handoff is in progress, the CU maintains the traffic channel with the losing node while establishing a new traffic channel with the gaining node. Once the new traffic channel is established, the CU drops the traffic channel with the losing node. The "make-before-break" handoff sequence is undesirable because the CU requires extra hardware to maintain the two simultaneous traffic channels.
A prior art "break-before-make" handoff sequence does not require a CU to maintain simultaneous traffic channels. When a handoff is in progress, the traffic channel to the losing node is dropped, the CU synchronizes to the gaining node, and a new traffic channel to the gaining node is established. Synchronization before traffic packet transmission is necessary in a mobile system because the Doppler offset and propagation delay between the CU and the gaining node requires the CU to adjust its packet transmission frequency and time prior to sending traffic packets. The Doppler offset and propagation delay are determined during synchronization and, because both vary during operation, also during maintenance of a traffic channel.
A drawback to the "break-before-make" handoff sequence is that a condition referred to as "drop-out" occurs during the time period when the CU is synchronizing with the gaining node. During drop-out, voice or data traffic packets from the CU are not received by any system node. Thus, a break in the voice or data transmission stream results. This break results in diminished voice quality and/or corrupted data.
What is needed is a method and apparatus for handing off a CU from a losing node to a gaining node which does not require additional CU hardware in order simultaneously to maintain multiple traffic channels. Further needed is a method and apparatus for handing off a CU which does not result in dropped CU voice and/or data packets.